The present invention generally relates to an apparatus known as digital still cameras and video cameras. More specifically, the present invention is directed to an imaging apparatus equipped with a function capable of preventing deteriorations of image qualities.
Since there are such needs that imaging apparatuses such as digital still cameras and video cameras can be easily carried out and also can be simply operated, housings of these imaging apparatuses have been made compact. In addition to the above-described requirements, since users have made strong requests for achieving high image qualities, in particular, higher resolution of photographed images has been progressed by increasing total pixel numbers of imaging elements. However, in order to achieve photographed images having high resolution, there are very important aspects, namely, not only larger pixel numbers of imaging elements, but also, higher precision required for optical performance of lens units employed in these imaging apparatuses.
On the other hand, there are such essential points that the housings of the imaging apparatuses are necessarily required to be made compact, and also, the lens units are necessarily required to be made compact. However, it is practically very difficult to realize the high precision for the optical performance of the lens units as well as the compactnesses of the lens units at the same time. As a consequence, deteriorations in the optical performance of the lens units, which are caused by making the lens units compact, may constitute such factors that image qualities of photographed images may be deteriorated.
As one of the above-described factors as to the deteriorations in the image qualities, which are caused by the deteriorations in the optical performance of the lens units, there is such a deterioration in the optical performance, which is caused by axial chromatic aberration of the lens units.
FIG. 7 is a diagram for illustratively showing an outline as to axial chromatic aberration which occurs in a lens unit. In this drawing, reference numeral 100 shows a lens unit; symbol “O” indicates an optical axis of the lens unit 100; symbol “LI” represents incident light; symbol “LR” is red light; symbol “LB” shows blue light; symbol “dR” indicates a focal length of the red light “LR”; and symbol “dB” represents a focal length of the blue light “LB.”
In this drawing, when the incident light “LI” is entered to the lens unit 100 in a parallel manner to the optical axis “O” of this lens unit 100, light passed through the lens unit 100 is collected on this optical axis “O.” However, since refractive indexes of the lens unit 100 are different from each other for the respective wavelengths of the light, light collected positions on the optical axis “O” are different from each other for the respective wavelengths of the light. Accordingly, light collected positions of the red light “LR”, the green light, and the blue light “LB” on the optical axis “O”, which are contained in the incident light “LI”, are made different from each other. As represented in this drawing, the light collected position of the red light “LR”, accordingly, the focal length “dR” of this red light “LR” becomes longer than the focal length “dB” of the blue light “LB”, becomes different from the light collected position of this blue light “LB.” Although the green light is not illustrated in this drawing, a focal length “dG” thereof becomes “dB<dG<dR”, so that this blue light is collected between the light collected position of the blue light “LB” and the light collected position of the red light “LR.”
More specifically, such a difference (namely, optical-axial chromatic aberration) of the light collected positions on the optical axis “O” with respect to the respective wavelengths of the light may give an adverse influence to high frequency components of the respective R, G, B color signals. As a result, such phenomena, colors different from original colors of an image of a photographing object (namely, false colors) and chromatic blur, may occur in particular at an edge portion of a photographed color image.
FIG. 8 is explanatory diagrams for explaining false colors and chromatic blur, which occur at an edge portion of an image signal and which are caused by axial chromatic aberration of the lens unit 100.
At the edge portion of the image signal, high frequency components appear in respective color signals of this image signal. FIG. 8 represents one example as to the high frequency components of these color signals at this edge portion.
Now, in FIG. 7, in such a case that an imaging plane of an imaging element (not shown) is set at such a position where the red light “LR” is focused on the optical axis “O”, namely, at the position of the focal length “dR” where the red light “LR” is collected, the lens unit 100 is just focused on a red color component of the image of the photographing object, so that a light amount characteristic of this red color component at a pixel position of an edge portion on the imaging plane of the imaging element may constitute such a characteristic of a very steep and large light amount, as represented in FIG. 8(a). In contrast to the above-described light amount characteristic, as to a blue color component of the image of the photographing object, since the imaging plane of the imaging element is shifted from the position of the focal length “dB” of the blue light “LB”, the lens unit 100 is not just focused on this blue color component. Thus, as represented in FIG. 8(c), a light amount characteristic of this blue color component at the pixel position of the above-described edge portion becomes such a characteristic that a small light amount is expanded to peripheral pixels and is made flat. Also, as to a green color component of the image of the photographing object, as represented in FIG. 8(b), such an intermediate light amount characteristic between the light amount characteristic indicated in FIG. 8(a) and the light amount characteristic indicated in FIG. 8(c) may be obtained in a similar manner.
As can be understood from the foregoing descriptions, when this imaging element is set at the position of the focal length “dR” of the red light “LR” of the lens unit 100, a signal having a red color component (will be referred to as “R signal” hereinafter) acquired from the imaging element becomes such a signal which contains a steep and high gain (level) of a high frequency component. However, such a high frequency component contained in a signal having a green color component (will be referred to as “G signal” hereinafter) has been attenuated and a waveform of this G signal has been made flat, while the G signal has been acquired by imaging a green color component of the photographing object image under such a condition that the imaging element is defocused. Also, such a high frequency component contained in a signal having a blue color component (will be referred to as “B signal” hereinafter) has been furthermore attenuated and a waveform of this B signal has been made further flat, while the B signal has been acquired by imaging a blue color component of the photographing object image under such a condition that the imaging element is furthermore defocused.
The R signal, the G signal, and the B signal, which are outputted from the imaging element, are processed based upon a predetermined signal processing operation, and thereafter, a color image is displayed by employing these processed R, G, B signals. When this color image is displayed, color light corresponding to the R signal, color light corresponding to the G signal, and color light corresponding to the B signal are added to each other, so that a color of a display image is represented with respect to each of pixels. However, if a plurality of color light are added to each other which are produced based upon high frequency components of the R signal, the G signal, and the B signal, which have been adversely influenced by the axial chromatic aberration by the lens unit 100 in the above-explained manner, then an edge portion of the display image is colored based upon such a color produced by adding the color light made of these high frequency components. In this case, the green light produced by the high frequency component of the G signal and also the blue light produced by the high frequency component of the B signal have been attenuated with respect to the red light produced by the high frequency component of the R signal, so that this edge portion is displayed by using such a color (namely, false color) which is different from the edge portion in the photographing object image. Also, as represented in FIG. 8(b) and (c), the high frequency component of the G signal and the high frequency component of the B signal have been made flat with respect to the high frequency component of the R signal. As a result, chromatic blur may occur in such a manner that colors are blurred from the edge portion.
The above-explained occurrence of the false color is related to such an imaging element that the three primary colors made of red, green, blue light are separated in the spectral manner, and then, the separated color light is photoelectrically converted so as to output the color signals. This technical idea may be similarly applied even in such an imaging element that incident light is separated into four complementary colors of magenta, cyan, yellow, and green light in the spectral manner, and then, the separated color light is photoelectrically converted so as to output color signals.
As previously described, since the axial chromatic aberration shown in FIG. 7 occurs in the lens unit 100, it is no possible that as to all of the red color light “LR”, the green color light “LG” (not shown in FIG. 7), and the blue color light “LB”, the lens unit 100 is just focused at the same time. As a result, as previously explained, in particular, the false colors and the chromatic blur occur at the edge portion containing the high frequency components, which may cause an image quality of a photographed image to be deteriorated.
Conventionally, various sorts of technical ideas have been proposed by which the chromatic blur at the edge portion caused by the axial chromatic aberration of such an imaging lens system may be corrected.
As one example of the conventional technical ideas, when color difference signals are produced from primary color signals, a difference in MTF (Modulation Transfer Function) characteristics between color components, which is caused by axial chromatic aberration, is converted in such a manner that the MTF characteristics are made coincident with such a color side that an image has been defocused. This conversion is carried out as follows: That is, for instance, a color difference signal “Cr” is produced by an R signal and a smoothing-processed G signal, namely, “<G> emphasized signal” by smoothing the G signal having the sharp (focused) green color component in order that the MTF characteristic of the green color component is approximated to the MTF characteristic of the red color component. Also, in a similar manner, another color difference signal “Cb” is produced by the B signal and this “<G> emphasized signal” by smoothing the G signal in order that the MTF characteristic of the green color component is approximated to the MTF characteristic of the blue color component. Thereafter, the G signal having the original MTF characteristic (which has not been smoothed) is added to the respective color difference signals “Cr” and “Cb” (refer to, for instance, JP-A-2007-28042).
This conventional technical idea is designed so as to avoid that the chromatic blur occurs which is caused by the axial chromatic aberration of the imaging lens system by performing the below-mentioned method: That is, the MTF characteristic of the R signal whose image has been defocused is matched with the MTF characteristic of the G signal, since the difference between the G signal having the original MTF characteristic and the G signal whose MTF characteristic is made equal to the MTF characteristic of the R signal is added to this R signal, and similarly, the MTF characteristic of the B signal is matched with the MTF characteristic of the G signal.